This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-346246 filed on Nov. 12, 2001.
The present invention relates an OFDM communication apparatus adopting an OFDM transmission technique for dividing data into pieces of data with different frequencies and transmitting the pieces of data completing the frequency division in parallel.
In recent years, attention is paid to the SDMA (Space Division Multiple Access) technology for improving a capacity of base station of accommodating user terminals through allocation of a space to a signal received from each of the user terminals by utilizing the directivity of an area antenna in an operation carried out by the base station to receive signals from the user terminals.
According to the SDMA technology, it is necessary to direct a beam in a direction toward a desired user terminal and direct a null point in directions toward user terminals other than the desired user terminal. A null point is a point from and to which no wave is to be received and transmitted. Normally, this technology is implemented by an adaptive array whose algorithm called the MMSE (Minimum Mean Square Error) algorithm is used in many cases.
With this technology, each user terminal adds a known signal determined in advance for each user terminal to a communication signal prior to transmission of the communication signal to a base station. When receiving such a communication signal, the base station determines a weight that minimizes a difference between the known signal added to the communication signal received from the user terminal and its replica, carrying out processing based on the weight.
In this way, a null point is created in a direction toward each of user terminals other than the desired terminal user to suppress communication signals received from the other user terminals. On the other hand, a reception beam is directed in a direction toward a desired one among user terminals in order to acquire a communication signal from the desired user terminal.
In order to apply the MMSE (Minimum Mean Square Error) algorithm to a base station adopting the OFDM technique, it is necessary to add a known signal to a communication signal as described above. There are the following two conceivable methods for adding a known signal to a communication signal.
(I) Insert a known signal in a time-axis direction of a sub-carrier.
(II) Insert a known signal in a frequency-axis direction of the sub-carrier.
A communication signal having such a configuration conforming to the OFDM technique is received by the base station along with a plurality of delay waves. If the delay time of each of the delay waves is found short in comparison with a period of time known as a guard interval in the reception of the communication signal, the base station will be capable of demodulating the communication signal without interferences of other symbols. This function results in two different suppression operations using the adaptive array for the above two conceivable means respectively.
In the case of the method (I) for inserting a known signal in a time-axis direction of a sub-carrier, if the delay time of each delay wave of the communication signal is found short in comparison with the guard interval, the adaptive array will not carry out a suppression operation on the delay waves. This is because, if the delay time of each delay wave of the communication signal is found short in comparison with the guard interval, a data symbol cannot be extracted well. Thus, the freedom of the antenna can be used for effectively suppressing other users. The freedom of the antenna is defined as a number capable of suppressing a signal. The freedom of the antenna is determined by the number of elements composing the antenna. Since the weight can be updated only once by one symbol of the OFDM technique, however, there is raised a problem of a long time it takes to determine a weight.
In the case of method (II) for inserting a known signal in a frequency-axis direction of a sub-carrier, on the other hand, in one operation to update a weight, data as much as sub-carriers can be used so that a weight can be determined within a short period of time. However, a delay wave distorting an OFDM spectrum inevitably becomes a target of suppression even if the delay time of the delay wave is shorter than the guard interval. Thus, a communication signal, which naturally does not have to be suppressed, is suppressed, causing freedom to be expended wastefully. That is, a communication signal, which naturally needs to be suppressed, cannot be suppressed in some cases. As a result, there arises a problem that, for each user terminal, a communication signal cannot be well separated from received signals.
It is thus an object of the present invention to provide an OFDM-technique communication apparatus capable of well separating a data sub-carrier signal transmitted by a predetermined terminal from data sub-carrier signals transmitted by all terminals.
According to the present invention, a communication apparatus receives communication signals from terminals by adoption of an OFDM technique. In the communication apparatus, the communication signal is received from each of the terminals. A Fourier transform process is performed on a known signal of the communication signal. The known sub-carrier signals are extracted from the communication signal. A Fourier transform process is performed on data signal of the communication signal. Data sub-carrier signals are extracted from the communication signal. A propagation route inference value is inferred for each of the extracted known sub-carrier signals according to the known sub-carrier signals. A combining weight used for carrying out a maximum ratio combining process on the data sub-carrier signals received by each of the antennas according to the inferred propagation route inference value for each of the terminals is found. One of the data sub-carrier signals that has been transmitted by a predetermined one of the terminals from the data sub-carrier signals transmitted by all of the terminals according to the computed combining weight is separated.